Timestamp problems for automake-based builds

[jrray]

As mentioned in the weekly meeting a few weeks ago, there is an issue when building packages that use automake and when using a tar source type (at least). When the build environment is rendered, the various files can have arbitrarily ordered timestamps which leads to make thinking some of the build files are out of date and need to be remade. This does not happen if the files retain their timestamps as found in the tarball.

It is also possible to avoid the problem by touching all the files with the same fixed timestamp, so a suggested fix is when rendering files to give them all the same timestamp of some fixed time (maybe the process start time of spfs-render). Actually saving timestamp info in the spfs object metadata is another possible route to take.